1. Field of the Invention
The present invention relates to an acoustic sensor and, more particularly, to an acoustic sensor formed on a semiconductor substrate.
2. Description of the Related Art
A capacitive silicon microphone is proposed as a semiconductor sensor for detecting acoustic vibration. In a microphone of this type, a diaphragm electrode and a backplate electrode are provided on a semiconductor substrate so as to form a capacitor. When sound pressure is applied to the microphone, the diaphragm electrode is vibrated. As the distance between the diaphragm electrode and the backplate electrode varying, the capacitance of the capacitor varies accordingly. Variation in voltage caused by the variation in capacitance is measured. The measured voltage represents an audio signal received by the microphone (See Reference (1) in the following Related Art List, for instance).
Related Art List
(1) Published Japanese translation of PCT International publication No. 60-500841.
A capacitive silicon microphone may be of smaller size and lighter weight than an elecret condenser microphone. The inventor of the present invention has come be aware of the following problem. The structural mechanical strength of a capacitive silicon microphone is likely to be impaired low due to its size smaller than that of the elecret condenser microphone. Further, a temperature cycle of a range between 400° and 800° is gone through every time a silicon nitride film or a silicon oxide film is deposited in the fabrication process. Therefore, a difference in stress is developed between the semiconductor substrate (a silicon substrate) and the diaphragm electrode. This results in internal stress and bending moment being developed in the diaphragm electrode, thereby reducing the sensitivity of the diaphragm. Reduction in sensitivity is also incurred due to capacitance around the diaphragm electrode and the backplate electrode. More specifically, the sensitivity corresponds to a value obtained by dividing the variation in capacitance caused by sound pressure by the overall capacitance. Ambient capacitance primarily acts to increase the overall capacitance and so practically leads to reduction in sensitivity.